Alternating current (AC) motor systems are used widely for: motor vehicles as their electric power steering systems, hybrid vehicles as their drive systems, and electric compressors as their drive systems. Such an AC motor system is designed to drive a multiphase motor having a multiphase stator coil by a multiphase inverter; this multiphase stator coil consists of star-connected multiphase stator windings.
As high-side switching elements and low-side switching elements (upper-arm switching elements and lower-arm switching elements) of the multiphase inverter, power semiconductor switching elements, such as IGBTs and/or MOSFETs, are normally used. In order to improve reliability, it is desirable for such AC motor systems to be designed to address open faults of the power semiconductor switching elements.
Japanese Patent Application Publication No. 2007-099066 discloses an electric power steering system for addressing open faults of power semiconductor switching elements constituting a three-phase inverter for driving a star-connected three-phase brushless motor.
The electric power steering system is provided with a first relay connected in series between a U-phase stator winding and a neutral point of the star-connected three-phase stator windings and a second relay connected in series between a V-phase stator winding and the neutral point of the star-connected three-phase stator windings.
The electric power steering system is also provided with a third relay connected in series between a W-phase stator winding and the neutral point of the star-connected three-phase stator windings, and a fourth relay connected between the neutral point of the star-connected three-phase stator windings and a neutral-point driver.
When a power semiconductor switching element for one phase stator winding fails, the relay corresponding to the one phase stator winding is turned off and the fourth relay is turned on. This allows the three-phase inverter to drive the remaining two-phase stator windings of the three-phase brushless motor. This driving method for driving the remaining two-phase stator windings will be referred to as “two-phase stator winding driving method” hereinafter.
However, the two-phase stator winding driving method may increase variations in the rotation of the three-phase brushless motor and those in the torque created thereby, as compared with driving of the three-phase stator windings of the three-phase brushless motor. This results in reducing the drivability of the three-phase brushless motor.
In addition, the electric power steering system disclosed in the Patent Application Publication may have a complicated circuit structure that individually turns off a power semiconductor switching element for each phase stator winding and turns on the fourth relay. This may increase the electric power steering system in size and/or manufacturing cost, resulting in reducing the installability of the electric power steering system in various vehicles and machines.